Add accessor methods for RTP timestamp of EncodedImage.

Intention is to make the member private, but downstream callers
must be updated to use the accessor methods first.

Bug: webrtc:9378
Change-Id: I3495bd8d545b7234fbea10abfd14f082caa420b6
Reviewed-on: https://webrtc-review.googlesource.com/82160
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24352}
diff --git a/modules/video_coding/frame_buffer.cc b/modules/video_coding/frame_buffer.cc
index b9241b9..9c2819e 100644
--- a/modules/video_coding/frame_buffer.cc
+++ b/modules/video_coding/frame_buffer.cc
@@ -87,7 +87,7 @@
   if (kStateEmpty == _state) {
     // First packet (empty and/or media) inserted into this frame.
     // store some info and set some initial values.
-    _timeStamp = packet.timestamp;
+    SetTimestamp(packet.timestamp);
     // We only take the ntp timestamp of the first packet of a frame.
     ntp_time_ms_ = packet.ntp_time_ms_;
     _codec = packet.codec;
@@ -213,7 +213,6 @@
 void VCMFrameBuffer::Reset() {
   TRACE_EVENT0("webrtc", "VCMFrameBuffer::Reset");
   _length = 0;
-  _timeStamp = 0;
   _sessionInfo.Reset();
   _payloadType = 0;
   _nackCount = 0;